Solvation and Cation Competition in Mixed Hydroxide Brines

Highly alkaline brines comprising mixtures of alkali metal cations are an important component of safe, aqueous chemistries for (long-duration) energy storage, electrowinning for the direct reduction of metal oxides to metal, industrial electrolysis, and many other technological applications. Physicochemical studies of the ion association, solvation dynamics, and transport in this highly concentrated regime are sparse, particularly with LiOH as a mixture component approaching its saturation limit, and could provide key inputs for physics-based modeling efforts and defining operational limits. To this end, this study maps the composition space for KOH/LiOH and KOH/NaOH ranging from total ionic strengths of 1 to 9 M, with a series of alkali cation mixtures at each ionic strength and an emphasis on contrasting the solvation dynamics and transport of brines with Li⁺ cocations versus Na⁺. Combining NMR chemical shift, diffusivity, ionic conductivity, density, viscosity, and NMR relaxation measurements yields a detailed understanding of the hydroxide affinity of Li⁺ compared to K⁺ and Na⁺ as well as the evolution of the solution structure as the Li⁺ saturation limit is approached, culminating in the elucidation of a distinct solvation regime in these mixed cation electrolytes at ionic strengths above 6 M for both KOH/LiOH and KOH/NaOH.